Hydroprocessing can include processes which convert hydrocarbons in the presence of hydroprocessing catalyst and hydrogen to more valuable products. Hydrocracking is a hydroprocessing process in which hydrocarbons crack in the presence of hydrogen and hydrocracking catalyst to lower molecular weight hydrocarbons. Depending on the desired output, a hydrocracking unit may contain one or more fixed beds of the same or different catalyst. Slurry hydrocracking is a slurried catalytic process used to crack residue feeds to gas oils and fuels.
In many regions naphtha is useful for motor fuel and petrochemical feed stock and its further recovery is desirable. Due to environmental concerns and newly enacted rules and regulations, saleable fuels must meet lower and lower limits on contaminates, such as sulfur and nitrogen. New regulations require essentially complete removal of sulfur from gasoline. For example, the U.S. Environmental Protection Agency is promulgating Tier 3 Motor Vehicle Emission and Fuel Standards in 2017 which limits gasoline sulfur to 10 wppm.
Hydrotreating is a hydroprocessing process used to remove heteroatoms such as sulfur and nitrogen from hydrocarbon streams to meet fuel specifications and to saturate olefinic or aromatic compounds. Hydrotreating can be performed at high or low pressures, but is typically operated at lower pressure than hydrocracking. Hydrodesulfurization is a hydrotreating process aimed at removing sulfur from the hydrocarbon by converting sulfur to hydrogen sulfide.
Hydroprocessing recovery units typically include an array of separators for cooling and depressurizing hydroprocessed effluent and separating gaseous streams from liquid streams and a stripping column for stripping hydroprocessed liquid with a stripping medium such as steam to remove unwanted hydrogen sulfide. The stripped stream then is typically heated and fractionated in a product fractionation column to recover products such as naphtha, kerosene and diesel.
In a refinery, hydrogen has a prime importance and recovery of hydrogen improves refinery profitability significantly. Pressure swing absorption (PSA) units are useful for purifying hydrogen by adsorbing larger molecules from the hydrogen stream at high pressure and then releasing the larger molecules at swing to lower pressure to provide a tail gas stream.
There is a continuing need, therefore, for improved methods of recovering low sulfur naphtha from hydroprocessed effluent streams.